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BA-3 As Preamp
BA-3 As Preamp
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Old 10th March 2014, 02:38 AM   #541
ammel68 is offline ammel68  United States
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Quote:
Originally Posted by 6L6 View Post
Essentially yes. The Jfets self-bias, as based on their Idss, and the Mosfets bias via the pots.

You an change the bias of the Jfet by changing the source resistance, but it's easier to just use them in the proper Idss range - anyting from 7.0-11.0 will work with no issues.
So if you have Jfets with a Idss of say 6.5mA for example, does that mean you will automatically read 65mV across R3 and R4?

If 8-10mA is recommended for passing through Q1 & Q2, shouldn't you be reading 80-100mV across R3 and R4?

Is it even possible to bias a Jfet with a Idss of 6.5mA to 8mA by lowering the source resistors below 10 ohms?
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Old 10th March 2014, 08:56 AM   #542
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whenever you introduce resistor in Jfet's source , while prior to that - gate and source were on same voltage potential , result is less current then Idss of part

just take Jfet as small pentode ....... or read this , for instance Junction Field Effect Transistor or JFET Tutorial
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Old 10th March 2014, 09:05 AM   #543
impuls is offline impuls  Yugoslavia
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I just finished my BA-3. I'm using 220 ohms for R13 and still have almost too much gain. You may want to try 180 ohms.
Look at post 495
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Old 10th March 2014, 02:29 PM   #544
ammel68 is offline ammel68  United States
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Look at post 495
OK, I did read post 495. I'm not sure what you mean by "out to ground 170 ohm".
Are you referring to R12?

The resistors are all numbered in the schematic, why not just state which resistors you changed like R10 & R11 to 65 ohms, R13 to 200 ohms, etc.?

I assume your combination resulted in much lower gain?
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Old 10th March 2014, 03:21 PM   #545
impuls is offline impuls  Yugoslavia
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R10, R11 65 ohms, R13 170 ohms, R6, R7 200 ohms.
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Old 10th March 2014, 03:40 PM   #546
ammel68 is offline ammel68  United States
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Thank you for clarifying which resistors you changed.
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Old 13th March 2014, 08:56 PM   #547
6L6 is offline 6L6  United States
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Check this out -- Very nice writeup of a whole club who made the BA-3 FE their reference preamp.

https://sites.google.com/site/audios...3-preamplifier
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Old 13th March 2014, 08:59 PM   #548
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Check it out! Wow! That's so cool!
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Old 13th March 2014, 09:12 PM   #549
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Old 13th March 2014, 09:15 PM   #550
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Vince - I also placed the 47K resistor in the R2 position and it got rid of the stepped attenuator clicks & pops. Thanks for mentioning it!
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